Thermal printheads
Thermal heads, as the name suggests, use heat to create the droplet. Used by manufacturers such as Canon and HP (not Epson), this method is commonly referred to as bubble jet. In a thermal inkjet printer, small resistors create heat. The heat vaporizes ink to create the bubble. As the bubble grows, some of the ink is pushed out of a nozzle onto the substrate. When the bubble pops (collapses), a vacuum is created, pulling more ink into the printhead from the cartridge.
A typical bubble jet printhead has 300 or 600 tiny nozzles, and all of them can fire droplets simultaneously. The inks used are usually water-based (aqueous) and use either pigments or dyes as the colorant. Latex ink is in the aqueous category. The inks used must have a volatile component to form the vapor bubble (water), otherwise droplet ejection cannot occur.
The advantages of thermal inkjet technology include the potential for very small drop sizes and high nozzle density. High nozzle density leads to compact devices, lower printhead cost and the potential for high native print resolution. The disadvantages of the technology are primarily related to fluid limitations. Not only does the ink have to contain a material that can be vaporized (usually an aqueous or part-aqueous solution), but it must withstand the effects of ultra-high temperatures. Because no special materials are required, the printhead is generally cheaper to produce than in other inkjet technologies.
Relating to cleaning and clearing the head, the thermal head can use significant force to create a spurt to keep the head open. The piezo head does not clear in this manner.
Piezoelectric, patented by Epson, uses piezo crystals. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate (resonate). When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls more ink into the reservoir to replace the ink sprayed out. When nozzles refuse to fire, a technician may ask you to increase the voltage to the particular color/head.